1. Field of the Invention
The invention lies in the field of coal desulfurization.
2. Prior Art
While the United States has very large known deposits of coal, many of these deposits are not now mined and the coal utilized because its content of sulfur is so high that when the coal is burned the excessive sulfur dioxide released to the atmosphere is far above permissible environmental standards. As a result, expensive transportation costs are incurred in shipping low sulfur content foreign coal into the eastern United States and shipping low sulfur content coal from the western United States to the eastern United States. The Coal existing in the eastern United States has such a high sulfur content that its use is substantially prohibited by environmental standards without desulfurization. Low sulfur content foreign oil is being imported into the United States in huge quantities as a source of energy which could otherwise be met if low sulfur content coal were readily available. The availability of an economically feasible process for substantially diminishing the sulfur content of the abundant high sulfur coals in the United States would have tremendous beneficial effect in decreasing this country's dependency on foreign oil and in decreasing the cost of coal for use by utilities in generating power. Obviously, the development of such a process is now the subject of an extensive and widespread research effort.
Two approaches to reducing the high sulfur content of domestic coals have been taken. One has been to provide large and expensive scrubbers to collect the sulfur dioxide from the stack gases following combustion. Such scrubbers are both expensive to build and to operate, and the sludges collected can create water pollution problems.
The second approach has been the desulfurization to the coal, either by mineral dressing to remove as much as possible of the coal away from pyrite or other inorganic sulfur minerals, or by a chemical attack on the inorganic sulfur and the organic sulfur. This latter approach is exemplified by the process described in Chemical and Engineering News, July 7, 1975, called "Battelle Hydrothermal Process". In this process, finely divided coal is treated in an autoclave with sodium hydroxide to react the latter with the pyritic sulfur and a substantial portion of the organic sulfur. While a substantial improvement over earlier processes, this process involves a complex technique to recover sulfur and regenerate reagents and is consequently expensive.
It has been known for some time that chlorine or a chlorine donor such as sulfur monochloride could be effective in chlorinating iron sulfides. In U.S. Pat. No. 2,895,796, C.T. Hill teaches the chlorination of pyrite with chlorine in a liquid sulfur bath. Peters, in U.S. Pat. No. 3,652,219, points out the problems of Hill's process with sulfur viscosity and discloses the chlorination of pyrite in a bath of sulfur monochloride.
Both processes have an inherent problem in that one of the primary reaction products can be ferrous chloride which melts at 670.degree. C. and is little soluble in either sulfur or sulfur monochloride. In coal, where the pyrite occurs in thin seams, the penetration of the lixiviant is prevented and effective desulfurization prevented. Indeed, neither process has been adopted for the removal of pyritic sulfur from coal.